The invention herein described was made in the course of work performed under a contract with the Department of the Army.
This invention relates to a method and apparatus for determining the suitability of liquid-solid metal compositions for casting.
Prior to the present invention, solid metal compositions have been prepared comprising discrete degenerate dendrite metal particles homogeneously distributed throughout a secondary metal solid. These compositions and methods for their preparation are described in copending U.S. patent application Ser. Nos. 379,991 and 379,990, filed July 17, 1973, each assigned to the assignee of this application. U.S. application serial Ser. No. 379,991 is a continuation in part of Ser. No. 258,383, filed May 31, 1972 and Ser. No. 153,819, filed June 16, 1971. U.S. application Ser. No. 379,990 is a continuation in part of Ser. No. 278,457, filed Aug. 7, 1972. All of these applications are incorporated herein by reference. As described in these applications, solid metal compositions are prepared comprising "primary solids" and "secondary solids" optionally containing third phase particles. These compositions can be reheated to form liquid-solid compositions which can be cast.
These compositions are prepared by heating a metal or metal alloy, which when frozen from a liquid state forms a dendritic network, to a temperature at which most or all of the metal composition is in a liquid state and vigorously agitating the composition to convert any solid particles therein to degenerate dendrites or nodules having a generally spheroidal shape. The agitation can be initiated either while the metallic composition is all liquid or when a small portion of the metal is solid, but containing less solid than that which promotes the formation of a solid dendritic network. Agitation can be continued with cooling and continued or can be initiated after cooling is initiated. If third phase particles are to be added to the metallic composition, they are added after all or a portion of the primary solids have been formed and they are dispersed within the metallic compositions such as by agitation. The resultant composition then can be cooled to form a slug which can be formed or cast subsequently by heating and shaping.
By the term "primary solid" as used herein is meant the phase or phases solidified to form discrete degenerate dendrite particles as the temperature of the melt is reduced below the liquidus temperature of the alloy into the liquid-solid range prior to casting the liquid-solid slurry formed. By the term "secondary solid" as used herein is meant the phase or phases which solidify from the liquid in the slurry at a lower temperature than that at which the primary solid particles are formed after agitation ceases. The primary solids obtained differ from normal dendrite structures in that they comprise discrete particles suspended in the remaining liquid matrix. Normally solidified alloys, in the absence of agitation have branched dendrites separated from each other in the early stages of solidification, i.e., up to 15 to 20 weight percent solid, and develop into an interconnected network as the temperature is reduced and the weight fraction solid increases. The structure of the compositions cast in accordance with this invention prevents formation of the interconnected network by maintaining the discrete primary solids separated from each other by the liquid matrix even up to solid fractions of 60 to 65 weight percent. The primary solids are degenerate dendrites in that they are characterized by having smoother surfaces and less branched structure which approaches a spherical configuration than normal dendrites and may have a quasi-dendritic structure on their surfaces but not to such an extent that interconnection of the particles is effected to form a network dendritic structure. The primary particles may or may not contain liquid entrapped within the particles during particle solidification depending upon the severity of agitation and the period of time that particles are retained in the liquid-solid temperature range. However, the weight fraction of entrapped liquid is less than that existing in a normally solidified alloy at the same temperature without agitation.
The secondary solid which is formed during solidification from the liquid matrix subsequent to forming the primary solid contains one or more phases of the type which would be obtained during solidification of a liquid alloy without vigorous agitation. That is, the secondary solid can comprise dendrites, single or multiphase compounds, solid solutions, or mixtures of dendrites, compounds and/or solid solutions.
As set forth above, the composition cast in accordance with this invention optionally can contain third phase solid particles homogeneously distributed within the primary solid-secondary solid matrix. The third phase particles are incorporated in the primary solid-secondary liquid slurry by adding them to the slurry and agitating the resultant composition until the third phase particles are dispersed homogeneously. The third phase particles have a surface composition that is either wet or not wet by the liquid portion of the slurry to which it is added. As employed herein, third phase particles that are wet refer to compositions which, when added to a metal or metal alloy at or slightly above the liquidus temperature of the metal or metal alloy and mixed therein, as by agitation with rotating blades, for a suitable period to effect intimate contact therewith, e.g., about 30 minutes, are retained in measurable concentrations within the liquid after agitation thereof has ceased and the resultant composition is allowed to return to a quiescent state when the metal or metal alloy is at or slightly above the liquidus temperature. When third phase particles are incorporated into a metal or metal alloy which wets the particles at the liquidus temperature of the metal or metal alloy, the particles are retained therein in concentrations from a measurable concentration of slightly above the 0% by weight, and generally up to about 5% by weight. Third phase particles having a surface composition that is not wet by the liquid metal is not retained homogeneously in measurable concentrations within the liquid after the agitation thereof has ceased and the resultant composition is allowed to return to a quiescent state.
In forming the primary solid-secondary solid compositions, the agitation employed is sufficient to prevent formation of interconnected dendritic networks or to substantially reduce of eliminate dendritic networks already formed on the solid particles.
One commercially important method for shaping metallic compositions containing primary solids comprises forming discrete slugs or blanks of the composition having a volume approximately that of the desired final cast article. The slug is cooled to a solid and stored until it is to be reheated and cast. In reheating the slug, it is necessary that the fraction solid present in the slug during casting be closely controlled. When the fraction solid is too high, the composition will fail to fill the mold cavity. When the fraction solid becomes too low, the desired properties of the composition containing primary solids will be substantially changed. That is, metallic compositions containing primary solids are thixotropic so that during periods of no agitation (when strain rate is zero) they exhibit a high shear stress, while during agitation, they exhibit shear thinning (shear stress decreases rapidly with increased strain rate) and therefore the composition can be cast when containing high fraction solids of up to about 60 to 65 weight percent solids. When cast in this state, the mold is subjected to far less heat as compared to that when the composition is cast when liquid so that the life of the mold is greatly increased when casting the compositions containing primary solids. Thus, in this respect, it is desirable to cast these compositions containing as high fraction solids as possible. Furthermore, compositions containing the higher fraction solids entrap little or no gas when being cast which is not the case with liquid compositions or compositions containing fraction solids. In addition, with compositions containing third phase particles, the increased liquid fraction and decreased fraction primary solids resulting from excessive reheating may cause the solid third phase particles to become segregated within the composition resulting in a cast article having nonhomogeneous physical characteristics.
Attempts to monitor the desired degree of reheating solely by monitoring the temperature of the reheated slug have not been satisfactory. With alloys, particularly those which melt over a narrow range, it is difficult to obtain reproducible results to obtain slugs having a fraction solid within the desired high range. Furthermore, even if a procedure for relating measured slug temperature to fraction solid in the slug were available, its cost and the time necessary to obtain desired accuracy would be incompatible with normal operating procedures existing in present production procedures. In addition, with higher temperature materials such as steel, thermocouples deteriorate rapidly and are expensive.